Mark Daly, 34, is BBC Scotland's Investigations Correspondent. For the past five months he has been writing about his preparation for the Barcelona Challenge - an Ironman distance triathlon: a 2.4 mile swim, followed by a 112 mile cycle, finished off by running a marathon - 26.2 miles. After two thousand training miles, half a dozen preparation races and no small amount of panicking, the big day was finally upon him. .

By Mark Daly

Mark Daly crosses the finish line at the Ironman race

Picture it. You've been racing for six hours; knocked out a 2.4 mile swim and more than 100 miles on a bike.

The heat is blistering and there's not a breath of wind. Your legs are screaming "no more" but there's only 10 miles left. Only 10 miles to the finish?

No. To the start of the marathon.

Well, that was me, on Sunday, at the Challenge Barcelona Ironman distance triathlon - and it was at this point I was sure I didn't have it in me.

Let's rewind a bit. I arrived in Calella, Maresme, about 40km outside Barcelona on Thursday, to acclimatise, get to know the course and to try to relax before the event.

There were 2,000 lean and mean looking triathletes in town, most looking like they had trained harder and longer than me and all with better bikes.

My own personal support team - my wife - had travelled with me and was engaged in trying to keep my panic and hypochondria down to a minimum.

Poor Seonaid has been forced to listen to my non-stop drivel about the race for so long, she is now well-versed in triathlon jargon and played a key role on the big day.

Regular readers will know that I've put in a fair amount of training hours for this and took part in several preparation triathlons, including two half Ironman distance events.

I probably averaged about 10-14 hours training a week for six months, give or take a few weeks off for a honeymoon and a fortnight off with the flu three weeks before the race.

It wasn't as much as I wanted, but I was still the best shape of my life and petrified by the challenge ahead.

A couple of bowls or pasta and a couple of beers to help me sleep, and it was off to bed.

The alarm went off at 5am. My stomach was churning. I had never known nerves like this.

I hadn't trained for more than seven hours in any one day - most experts say you shouldn't - so I had no way of knowing how my body would respond eight, nine or ten hours into the race.

The run is my weakest event and my only other marathon attempt was such a disaster that I still don't like to talk about it.

Mark finished the race in a time of 10 hours 54 minutes and 8 seconds

The truth is, after all this fuss, I was just scared of blowing up and having to tell people I didn't finish.

I was off in the fifth heat at 7.50am, and it was to be a fast wave packed with 400 quick guys.

As I lined up on the beach I took a few seconds to savour the moment, caught a glimpse of Seonaid in the crowd before the hooter went, and we were off!

The 3.8km swim is supposed to be my strongest leg, but with up to another 11 hours of racing ahead of me, I decided to take it steady.

As I hauled myself out of the water, my watch said 61 minutes. Bang on target, and sitting in 152nd place overall.

With the shouts from the crowd ringing in my ears, I raced up to transition and struggled out of my wetsuit.

I picked out my bike and set off. 180km of smooth Spanish roads lay ahead, with the bike leg split into three laps of 70, 70 and 40km.

On the first lap I completely ignored my own strict instructions not to push too hard, averaging about 34 kmph. Deep down I knew I would pay for this later.

The next lap was a bit more conservative. I took advantage of every aid station, which came approximately every 20km.

As I got within 20km of the final transition, the enormity of my predicament started to grip me.

How could I possible get off this bike and run a marathon? In this 30 degree heat?

The thought of it was eating me up and I was actually contemplating what my excuses could be for bombing out… a puncture, cramp, sabotage. Anything.

Then I tried to get a grip of myself. Maybe I could do it after all…

I finished the bike leg in 5 hours 35mins, 645th fastest overall - and all 644 of them seemed to whizz past me.

As I started running, to my amazement, I felt okay.

"Hang on a minute, I can do this," I said to myself as I started out on the 42.2km marathon course. Oh how I would pay for that optimism.

The first two 10km laps went according to plan. Then it all started to go wrong. One of my little toes started to annoy me. Then it was hurting.

Mark with his supportive wife, Seonaid

Next there was blood oozing out from my trainer but I was too scared to take it off and see what the problem was in case it was so serious that I'd have to stop.

In any case, that particular pain was being well outplayed by the general agony the rest of my body was enduring, especially my knees and back.

By now I was a bit delirious anyway and was only ever thinking about the next aid station, 2.5km away.

Cold sponges, water and Coke awaited. I even stopped for a few seconds for a kiss from my wife to see if that helped. Nope.

On the last lap, with 10km to go, and the golden sub 11-hour target still in sight, I got a bit of a boost and felt strong for a couple of kilometers.

But that soon passed and a blurry anguish was upon me again.

The last stretch felt like it took 100 years, but as I entered the final 300m and the crowd thickened, all my pain and discomfort disappeared and I started bounding along, high fiving the vociferous Spaniards as I went.

Into the finishing chute and I could see the line.

I whooped and shouted and as I summoned every last drop of energy to sprint over it only to fall into the arms of the race volunteers.

I felt sick, weak and dizzy, but totally elated. My marathon time was 4 hours 9 mins, 468th fastest.

Seonaid managed to sneak into the athletes area to give me a hug and hear my tales.

Out of 1,900 triathletes, I had finished 379th in a time of 10 hours 54 minutes and 8 seconds - about 2 hours 40 mins behind the winner.

It's hard to describe, and only those who have been there can truly understand, but despite the pain - and I have never known pain like it - this was a memorable and wonderful day in my life. One of my proudest.

Will I ever do another Ironman distance triathlon?

No way.

I said right form the start this was a once in a lifetime thing.

And I meant it.

Really. Absolutely, definitely never again.

Well, probably never.

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